Along with progress of Internet technologies, there are increasing opportunities to save an image captured with the use of an image-capturing apparatus-equipped mobile terminal apparatus such as a mobile phone or a smartphone having a camera function. In addition, there are increasing opportunities to capture not only images of landscapes, portraits, etc. but also images of explanatory diagrams and texts displayed in exhibitions etc. and images of slides used in academic conferences etc. A captured image is saved in a mobile terminal apparatus while the captured image is provided with a file name automatically given by the mobile terminal apparatus based on information such as an image-capturing date or a file name set by a user.
Such a captured image includes not only a captured image obtained by capturing an object from a front but also a captured image obtained by capturing the object from an oblique direction. The captured image obtained by capturing the object from the oblique direction has distortion. Therefore, for example, a technique is disclosed for carrying out a geometry distortion correction so as to generate an image which is prevented from having distortion. In order to generate an image which is prevented from having distortion, for example, Patent Literature 1 discloses a photographing apparatus which finds a contour of an image of an object, finds a shape of the image on the basis of the contour, finds a projection parameter showing a relationship between the shape of the image and an actual object, and carries out image conversion with the use of this projection parameter.
Meanwhile, there is a technique for converting an image into a compact PDF file (generating a compact PDF file, causing an image to be in a compact PDF format). An image compression technique based on layer separation such as Mixed Raster Content (MRC) has been practically used as a technique for use in converting an image into a compact PDF file. According to the image compression technique based on layer separation, a foreground mask indicative of a text and/or a line art image is generated from an image to be compressed, a color image is separated into a foreground layer and a background layer based on the generated foreground mask, and the foreground layer and the background layer are compressed with the use of compression techniques suitable for the foreground layer and the background layer, respectively. A compact image is finally generated through this processing.
Note here that a foreground layer, which is a layer including a text and/or a line art image, is generally compressed with the use of a lossless compression technique such as JBIG (Joint Bilevel Image Group), MMR (Modified Modified Read code), or LZW (Lempel Ziv Welch). Meanwhile, a background layer, which is a layer indicative of an image other than a text and/or a line art image, is generally compressed with the use of a lossy compression technique such as JPEG (Joint Photographic Experts Group).
A technique for converting an image into a compact PDF file is normally applied to an image obtained by reading a document placed on a contact glass, or an image obtained by reading a document conveyed from a document conveying apparatus. According to such a reading system, a distance between a reading apparatus and a document surface is fixed. This prevents an image obtained by reading a document from being blurred, and prevents the document from being read with the document excessively skewed.